The present invention relates in general to a dry, solid, water-swellable, water-insoluble absorbent composition of matter, and more particularly to such a composition of matter which provides a gelatinous agglomerate of liquid - swollen particulate members in the presence of a quantity of body exudate, is capable of absorbing at least 15 times its weight in body exudate, and is capable of retaining the absorbed exudate when exposed to pressure sufficient to deform the agglomerate.
As evidenced by such recent patents as U.S. Pat. Nos. 3,628,534, 3,699,103, and 3,670,731, there has been a high degree of activity in the area of water-insoluble particulate hydrocolloid absorbent compositions of matter and products using the same, such as absorbent dressings, diapers, catamenial tampons, and the like. Such compounds maintain their particulate character as they imbibe and absorb many times their weight of surrounding liquid, and in doing so swell. The compounds are capable of absorbing at least 15 times their weight of water, urine and other body exudates. In doing so, each individual absorbent particle swells or enlarges several hundred percent times its individual parameter without destruction of its initial particulate integrity. As the water-insoluble compound accepts liquid, it substantially immobilizes the liquid therein, and the resulting particulate liquid-swollen structure is gelatinous.
The mass of swollen particulate water-insoluble particles defines an aciniform structure since each individual absorbent particle is a greatly enlarged particle, having become liquid-swollen or grape-like or acinus in form due to the water, urine or other liquid it has absorbed. The individual swollen particles are tacky and hence form a clustered mass of liquid-swollen particles. The particles remain in this aciniform state even in the presence of liquid in excess of their ability to absorb.
The liquid-swollen particles bind their absorbed water tightly, but upon drying the particles are dehydrated and return more or less to their original size. At this time they can operate more or less as before to absorb and bind liquids.
The water-insoluble particles described above are generally formed through the polymerization of one or more monomers, which if homopolymerized would form a water-soluble polymer, with a monomer which covalently crosslinks the molecule and introduces a limited water-insolubility. In general the degree of crosslinking is contained so that the polymer network of the hydrocolloid is not soluble in water, urine and the like, yet remains flexible and swells as water and other liquid is absorbed within its structure. As the hydrocolloid swells, it maintains the approximate shape and geometry it had before contact with liquid, but the dimensions are greatly enlarged to provide for the binding of the liquid absorbed therein.
Such water-insoluble hydrocolloid absorbent compositions of matter represent an advance over the prior art watersoluble hydrocolloid materials which merely increase the viscosity of (i.e. thickens) the liquid exposed thereto. Such soluble hydrocolloids serve only to increase the viscosity of the liquid and, in the presence of an added liquid excess, lose their power to retain the viscosity they had previously achieved.
Nonetheless, even the water-insoluble hydrocolloid absorbent compositions described above have not proven to be entirely satisfactory, especially from the point of view of the manufacturer. The prior art water-insoluble absorbent compositions are typically formed by the incorporation into a monomer mixture to be polymerized of a known covalently crosslinking monomeric agent, typically a non-conjugated divinyl compound such as methylene bis acrylamide. As such covalent crosslinking agents actually entered into the formation of the linear polymeric backbone of the absorbent composition, post-polymerization addition of the crosslinking agent (i.e., addition of the cross-linking agent after formation of the linear polymer backbone) could not be utilized, and the manufacturer thus required full polymerization facilities rather than the more limited facilities required for merely adding a crosslinking gent to a prepolymerized mixture and effecting the desired crosslinking. Another disadvantage of the known water-insoluble hydrocolloid compositions is that the composition, once formed, had to be applied to the substrate (for example, the backing of an absorbent dressing) by techniques for handling solids rather than the simpler techniques available for handling liquids.
Accordingly, it is an object of the present invention to provide a water-insoluble absorbent composition of matter in which the water-soluble polymeric backbone may be rendered water-insoluble either during or after polymerization.
It is also an object to provide such a composition in which the various ingredients thereof may be applied as a solution to a substrate and easily converted in situ to the water-insoluble composition.